1. Field of the Invention
The present invention relates to exposure methods, exposure apparatus, and device manufacturing methods, and more particularly to an exposure method and an exposure apparatus that are used when exposing an object via a liquid, and a device manufacturing method that uses the exposure method and the exposure apparatus in a lithographic process.
2. Description of the Background Art
Conventionally, in a lithographic process to produce electronic devices such as a semiconductor (integrated circuit) or a liquid crystal display device, the reduction projection exposure apparatus based on a step-and-repeat method (the so-called stepper) that transfers an image of a pattern of a mask (or a reticle) via a projection optical system onto each of a plurality of shot areas on a sensitive object such as a wafer, a glass plate or the like (hereinafter generally referred to as a “wafer”), on which a resist (sensitive agent) is coated, the projection exposure apparatus based on a step-and-scan method by (the so-called scanning stepper (also called a scanner) and the like are mainly used.
With these types of projection exposure apparatus, a higher resolving power (resolution) is required year by year due to finer patterns according to higher integration of the integrated circuits, and in recent years, as a method for substantially shortening the exposure wavelength and also increasing (widening) the depth of focus compared with the depth of focus in the air, exposure apparatus that use the liquid immersion method are beginning to gather attention. As an exposure apparatus that uses the liquid immersion method, an exposure apparatus that performs exposure in a state where the space between the lower surface of a projection optical system and the surface of a wafer is filled with liquid such as water, an organic solvent or the like is known (e.g. refer to the pamphlet of International Publication WO99/49504 description). In the exposure apparatus according to Patent Document 1, the resolution is improved by the use of the fact that the wavelength of the exposure light in the liquid becomes 1/n of the wavelength in the air (n is the refractive index of the liquid which is normally around 1.2 to 1.6), and the depth of focus is also substantially enlarged n times when comparing it with the case when the same resolution is obtained without applying the liquid immersion method to the projection optical system (supposing that such a projection optical system can be made). That is, the depth of focus can be substantially enlarged n times than in the air.
However, in the liquid immersion exposure apparatus described earlier, various measurements related to exposure are performed in a state where an liquid immersion area is formed on an object (a member) on which the exposure light is irradiated.
Further, in the liquid immersion exposure apparatus, pure water or purified water is proposed as an example of the liquid. This is because purified water can be obtained in large quantities at a semiconductor manufacturing plant or the like, and it also has an advantage of having no adverse effect on the photoresist on the wafer or to the optical lenses or the like.
However, because purified water has high specific resistance, static electricity is generated due to friction with piping, nozzles or the like, and in the case the liquid immersion area is formed on the object (the member) with the charged purified water, there is a possibility that the object (the member) or a film on the surface of the object (the member) or both may be electrically charged. In this case, the film on the surface of the object (the member) on which the liquid immersion area is formed could be deteriorated or damaged. The deterioration of the film may cause unevenness in the optical properties of the deteriorated section and the section besides the deteriorated section, and/or may be the cause of water stains (water marks), which may consequently decrease the exposure accuracy. Further, foreign substances or foreign matters may adhere on the surface of the object (the member) by the charge, and such foreign substances may contaminate the liquid (purified water), the wafer and the like, which may cause faulty exposure such as defects.
Further, in the case the liquid immersion area is formed on the object (e.g. a wafer) subject to exposure with the charged purified water, the film (a resist layer, and/or a top coat layer) on the object (the wafer) could be charged. In this case, deterioration, modification or the like may occur on the film (a resist layer, and/or a top coat layer) on the object (the wafer), which could be the cause of defects being generated.
Further, in the case the object (the wafer) subject to exposure is charged, foreign substances may be adhered on the surface of the object (the wafer), and the liquid (purified water) and the object (the wafer) subject to exposure may be contaminated, which may cause faulty exposure such as defects.